def solution(a):
    d = {}
    for e in a:
        if (e in d):
            d[e]+=1
        else:
            d[e]=1
        
    for k,v in d.items():
        if(v%2==1):
            return k

a = [2, 2, 1, 3, 3, 1, 10]

print(solution(a))